1. Technical Field
This invention is generally directed to wireless communication, and more specifically, for improving polling-based communication systems by use of selective polling.
2. Background Art
Wireless systems, such as those compliant with the IEEE802.11 MAC protocol, typically have two functions for controlling channel access. One is the Distributed Coordination Function (DCF) based on algorithms such as the Carrier Sense Multiple Access with Collision Avoidance algorithm (CSMA/CA). The other is the Point Coordination Function (PCF). The DCF and PCF utilize various kinds of gaps between sequential transmission signals in order to avoid collisions for channel access. Transmission signals (packets) are organized into “frames,” and gaps between two adjacent frames are called IFS (inter frame spacing). Known examples of inter frame spacing include the Short Inter Frame Space (SIFS), the Priority Inter Frame Space (PIFS), and the Distributed Inter Frame Space (DIFS).
FIG. 1 illustrates conventional examples of a time-slotted channel hopping allocation process involving SIFS, PIFS and DIFS, respectively. In FIG. 1, SIFS is the shortest time slot and the PIFS is the second shortest time slot. The transmission after the shorter IFS has the highest priority. Thus, SIFS is normally used in connection with burst transmissions and response access, such as the ACK (acknowledge) signal. PIFS is used for priority access, such as use in connection with a beacon signal.
DIFS is used in connection with the DCF function, which is based on the CSMA/CA. A station (STA) obtains channel access based upon monitoring of the available communication media for an idle condition. The STA also is provided with a random backoff time before it starts sending a packet, in order to avoid conflicting with a packet transmitted by a higher priority STA.
In the PCF function, the Access Point (AP) obtains channel access after it monitors the available communications media for an idle condition based upon the PIFS. As PIFS is shorter than DIFS, the AP may have a higher priority than other STAs. Once the AP obtains channel access, the AP can give STAs a chance to transmit without contention by polling. For example, the AP can send a polling packet, or a Coordination Function Poll (CF-Poll) to a STA and the STA receiving the CF-Poll can transmit data to the AP after the SIFS. This arrangement provides a centralized mechanism that allows the AP to control STAs and provide high efficiency channel access.
The IEEE802.11e standard also provides a MAC extension of an existing IEEE802.11a/b/g MAC through two new mechanisms: Enhanced DCF channel access (EDCA) and Hybrid Coordination Function (HCF) Controlled Channel Access (HCCA). The EDCA extends the legacy DCF function to provide a service differentiation and prioritization mechanism. This mechanism is still based on CSMA/CA, but also introduces Arbitration IFS (AIFS) to make several levels of priority.
Although the EDCA function may provide a parameterized Quality of Service (QoS), it cannot provide adequate deterministic latency as long as it is based on CSMA/CA. Further, the reference scheduler design of the HCCA is not sufficient to ensure deterministic latency because it is not taking packet errors into account. For example, if a STA fails to transmit data to the AP, the STA would have to wait for the next polling packet, which comes with the next SI.